I. Field of the Invention
The present invention relates to a magnetic bubble memory device and, more particularly, to a magnetic bubble memory device having a structure suitable for a high packing density.
II. Description of the Prior Art
FIG. 1 is a sectional view of a magentic bubble memory device having propagation paths made of a soft magnetic film. As will be seen from FIG. 1, thicknesses of first and second insulating layers 2 and 4 are made uniform throughout the device.
The conventional device of the type described above is easy to manufacture, but its characteristics are disadvantageously restricted. For example, because of the provision of a conductive layer 3 beneath a propagation path 5, a portion of the second insulating layer 4 intervening therebetween may not be made very thin. In order to solve this problem, an approach has been made as disclosed in Japanese Preliminary Patent Publication No. 56-148784. According to this proposal, as shown in FIG. 2, a second insulating layer 4 is formed only on a conductive layer 3 for a gate and in the vicinity thereof. At the same time, element 5a on the conductive layer 3 of a magnetic bubble propagation path 5 of soft magnetic film has a pattern period larger than that of the other element 5b of the path 5. In FIGS. 1 and 2, reference numeral 1 designates a magnetic film made of garnet for holding magnetic bubbles.
However, there still exist several problems in considering the structure of a magnetic bubble memory device.
As shown in FIG. 3, a magnetic bubble memory device comprises a data storage section having a plurality of minor loops 10 and a major loop (major line) 11 in association with the plurality of minor loops 10 so as to read or write data. A gate 12 is formed to connect the major line 11 with each minor loop 10 and to control transfer-in or transfer-out of the magnetic bubbles.
The gate 12 has a complex structure which disables micropatterning. In a magnetic bubble memory device of high density, as shown in FIG. 3, each minor loop 10 is formed in a single or multiple folded shape, thereby providing sufficient space for the corresponding gate 12. In FIG. 3, reference numeral 14 designates a bubble detector and 15 a bubble generator.
With the above configuration, the period of the magnetic bubble propagation path pattern constituting the major loop 11 is larger than that constituting the minor loops 10. Typically, the period of the major loop 11 is about twice or four times that of the minor loops 10.
The present inventors have made extensive studies and have found that a distance l between the magentic bubble propagation path 5 made of a soft magnetic film and the magnetic film 1 for holding magnetic bubbles need not be minimized as possible, but must be kept at an optimum value throughout the device. The optimum value varies according to the propagation path pattern period as shown in FIG. 4. Referring to FIG. 4, curves a and b indicate characteristics for pattern periods of 6 .mu.m and 12 .mu.m, respectively. It is apparent that a propagation path pattern with longer period requires a larger optimum value of the distance l. When the distance l becomes too small, the attraction force between the magnetic bubble and the propagation path is excessively increased. As a result, the magnetic bubble cannot run across a gap between adjacent propagation path elements. In general, a bias magnetic field margin of 10% or more is required for the propagation path. Therefore, in a magnetic bubble memory device wherein the pattern period of the major line differs from that of the minor loops, it is not preferred that the distance between the propagation path and the magnetic film is increased only at the gate as in the aforementioned publication.